Graphite electrode comprising electrochemically reduced graphene oxide and methods thereof

ABSTRACT

A modified graphite electrode comprising a coating of electrochemically reduced graphene oxide. The modified graphite electrode may be employed in detecting of uric acid. A sensing device comprising the modified graphite electrode and a method of making the modified graphite electrode are described herein.

BACKGROUND OF THE INVENTION Field of the Invention

This disclosure relates to a graphite electrode with an outer surfacecoated with electrochemically reduced graphene oxide, a method of makingthe graphite electrode, a device comprising the graphite electrode, andelectrochemical quantification of analytes.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Graphene is a one-atom thick carbon sheet in which the carbon atoms aresp²-hybridized and arranged in a hexagonal lattice (X. Huang, Z. Yin, S.Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Small. 7 (2011)1876-1902, incorporated herein by reference in its entirety). Due to itsunique structural and physicochemical properties, it is widely used indifferent disciplines such as transparent conductors, energy storagedevices, field emission display and nano-electronics (Y. Liu, X. Dong,P. Chen, Chem. Soc. Rev. 41 (2012) 2283-2307, incorporated herein byreference in its entirety). Graphene also has an excellent mechanicalstrength which is approximately 200 times more than steel (M. Pumera, A.Ambrosi, A. Bonanni, E. Chng, H. Poh, Trends Anal. Chem. 29 (2010)954-965, incorporated herein by reference in its entirety). Graphene hasextraordinary electrochemical properties including fast charge transfer,wide potential window and less resistance to charge transfer (S. Wu, Q.He, C. Tan, Y. Wang, H. Zhang, Small. 9 (2013) 1160-1172; and N. Baig,A.-N. Kawde, Anal. Methods. 7 (2015) 9535-9541, each incorporated hereinby reference in their entirety). At room temperature, the electronmobility is 200000 cm² V⁻¹s⁻¹ and surface area is 2600 m²/g (T. Gan, S.Hu, Microchim. Acta. 175 (2011) 1-19, incorporated herein by referencein its entirety).

In view of the foregoing, one objective of the present disclosure is toprovide a modified graphite electrode comprising a coating ofelectrochemically reduced graphene.

SUMMARY OF THE DISCLOSURE

The foregoing description is intended to provide a general introductionand summary of the present disclosure and is not intended to be limitingin its disclosure unless otherwise explicitly stated. The presentlypreferred embodiments, together with further advantages, will be bestunderstood by reference to the following detailed description taken inconjunction with the accompanying drawings.

A first aspect of the disclosure relates to a modified graphiteelectrode, comprising a graphite electrode with an outer surface, and acoating on the outer surface of the graphite electrode, where thecoating comprises electrochemically reduced graphene oxide, and themodified graphite electrode has an electroactive surface area in a rangeof 0.3-0.6 cm².

In one embodiment, the graphite electrode has an electroactive surfacearea in a range of 0.04-0.07 cm².

In one embodiment, the graphite electrode is in the form of a pencillead.

In one embodiment, a thickness of the coating is in a range of 0.001-5μm.

In one embodiment, the pencil lead has a diameter in a range of 0.1-1.4mm, and a length in a range of 20-70 mm.

In one embodiment, the pencil lead is of a grade 6H, HB, B, or 4H.

A second aspect of the disclosure relates to a method for detecting uricacid, comprising: (i) immersing the modified graphite electrode of thefirst aspect, a reference electrode, and a counter electrode in anaqueous sample comprising 0.02-1,000 μM uric acid, and (ii) applying asquare wave potential to the aqueous sample.

In one embodiment, the reference electrode is Ag/AgCl, and the counterelectrode comprises platinum.

In one embodiment, the square wave potential has an amplitude in a rangeof 0.01-0.1 V, a frequency in a range of 10-100 Hz, and an adsorptiontime in a range of 10-240 s.

A third aspect of the disclosure relates to a device for sensing ananalyte in a sample, comprising a reference electrode, a counterelectrode, and the modified graphite electrode of the first aspect.

In one embodiment, the reference electrode is Ag/AgCl.

In one embodiment, the counter electrode comprises platinum.

The fourth aspect of the disclosure relates to a modified graphiteelectrode, comprising: (i) dispersing graphene oxide in a buffer to forma graphene oxide dispersion, (ii) immersing a graphite electrode with anouter surface, a reference electrode, and a counter electrode in thegraphene oxide dispersion, and (iii) applying a cyclic potential to thegraphene oxide dispersion to coat the outer surface of the graphiteelectrode with a coating comprising electrochemically reduced grapheneoxide thereby forming the modified graphite electrode.

In one embodiment, a concentration of the graphene oxide in the grapheneoxide dispersion is in a range of 1-5 mg/mL.

In one embodiment, the buffer is acetate buffer with a concentration ina range of 0.01-0.5 M.

In one embodiment, the reference electrode is Ag/AgCl.

In one embodiment, the counter electrode comprises platinum.

In one embodiment, the cyclic potential is applied at a scan rate of10-120 mV/s from −1.4 V to 0.3 V for 3-10 cycles.

In one embodiment, the graphite electrode has an electroactive surfacearea in a range of 0.04-0.07 cm², a length in a range of 20-70 mm, is inthe form of a pencil lead with a diameter in a range of from 0.1-1 mm,and of the type 6H, HB, F, B, or 4H.

In one embodiment, the modified graphite electrode has an electroactivesurface area in a range of 0.3-0.6 cm², and a thickness of the coatingis in a range of 0.001-5 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the shape of the ridges on an embodiment of the modifiedgraphite electrode.

FIG. 1B shows the collapsed shape of the ridges on another embodiment ofthe modified graphite electrode.

FIG. 1C shows the folded shape of the ridges on another embodiment ofthe modified graphite electrode.

FIG. 1D is a drawing of an embodiment of the modified graphiteelectrode.

FIG. 1E is a cross-sectional view of an embodiment of the modifiedgraphite electrode.

FIG. 1F is a Fourier transform infrared spectrum of graphite.

FIG. 2 is a Fourier transform infrared spectrum of graphene oxide.

FIG. 3 is a Raman spectrum of graphite.

FIG. 4 is a Raman spectrum of graphene oxide.

FIG. 5 shows a dispersion of graphite.

FIG. 6 shows a dispersion of graphene oxide.

FIG. 7 is a micrograph of a bare graphite electrode at 5,000×magnification.

FIG. 8 is a micrograph of a modified graphite electrode at 5,000×magnification.

FIG. 9 is an overlay of cyclic voltammograms at a bare graphiteelectrode in a solution comprising 5 mM K₃Fe(CN)₆/K₄Fe(CN)₆ and 0.1 MKCl with scan rates of 20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s, and 100 mV/s.

FIG. 10 shows a linear relationship between the peak current and thesquare root of the scan rate observed in FIG. 9.

FIG. 11 is an overlay of cyclic voltammograms at a modified graphiteelectrode in a solution comprising 5 mM K₃Fe(CN)₆/K₄Fe(CN)₆ and 0.1 MKCl with scan rates of 20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s, and 100 mV/s.

FIG. 12 shows a linear relationship between the peak current and thesquare root of the scan rate observed in FIG. 11.

FIG. 13 is an electrochemical impedance spectrum of a bare graphiteelectrode in a solution comprising 5 mM K₃Fe(CN)₆/K₄Fe(CN)₆ and 0.1 MKCl.

FIG. 14 is an electrochemical impedance spectrum of a modified graphiteelectrode in a solution comprising 5 mM K₃Fe(CN)₆/K₄Fe(CN)₆ and 0.1 MKCl.

FIG. 15 is a cyclic voltammogram (scan rate: 100 mV s⁻¹) at a baregraphite electrode in a solution comprising 1 mM uric acid in aphosphate-buffered saline (PBS) buffer (0.1 M, pH 7.0).

FIG. 16 is a cyclic voltammogram (scan rate: 100 mV s⁻¹) at a modifiedgraphite electrode in a solution comprising 1 mM uric acid in a PBSbuffer (0.1 M, pH 7.0).

FIG. 17 is an overlay of square wave voltammograms at a modifiedgraphite electrode in a solution comprising 10 μM uric acid in a PBSbuffer (0.1 M) at pH 8.0, 7.5, 7.0, 6.5, and 6.0.

FIG. 18 is a plot of the peak currents versus the pH values observed inFIG. 17.

FIG. 19 shows a linear relationship between the pH value and the peakoxidation potential observed in FIG. 17.

FIG. 20 shows a relationship between the current and the amplitude ofthe square wave potential at the modified graphite electrode in a samplecomprising 10 μM uric acid in a PBS buffer (0.1 M, pH 7.0).

FIG. 21 shows a relationship between the current and the frequency ofthe square wave potential at the modified graphite electrode in a samplecomprising 10 μM uric acid in a PBS buffer (0.1 M, pH 7.0).

FIG. 22 shows a relationship between the current and the absorption timeat the modified graphite electrode in a sample comprising 10 μM uricacid in a PBS buffer (0.1 M, pH 7.0).

FIG. 23 is an overlay of square wave voltammograms at the modifiedgraphite electrode in PBS buffer (0.1 M, pH 7.0) with variousconcentrations of uric acid: 0 μM, 0.2 μM, 0.5 μM, 2 μM, 6 μM, 10 μM, 14μM, 18 μM, and 22 μM.

FIG. 24 shows a linear relationship between the peak current and theconcentration of uric acid.

FIG. 25 is a square wave voltammogram of 500 μM ascorbic acid in thepresence of 50 μM uric acid with square wave voltammetry parameters:amplitude 0.02 V, frequency 25 Hz, and adsorption time 30 s.

FIG. 26 is a square wave voltammogram of 500 μM ascorbic acid withsquare wave voltammetry parameters: amplitude 0.02 V, frequency 25 Hz,and adsorption time 30 s.

FIG. 27 is a square wave voltammogram of 500 μM ascorbic acid in thepresence of 50 μM uric acid with square wave voltammetry parameters:amplitude 0.03 V, frequency 50 Hz, and adsorption time 120 s.

FIG. 28 is a square wave voltammogram of 500 μM ascorbic acid withsquare wave voltammetry parameters: amplitude 0.03 V, frequency 50 Hz,and adsorption time 120 s.

DETAILED DESCRIPTION OF THE DISCLOSURE

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

Within the description of this disclosure, where a numerical limit orrange is stated, the endpoints are included unless stated otherwise.Also, all values and subranges within a numerical limit or range arespecifically included as if explicitly written out. As used herein, thewords “a” and “an” and the like carry the meaning of “one or more”.

The first aspect of the disclosure relates to a modified graphiteelectrode comprising a graphite electrode 101 with an outer surface 102and a coating 103 on the outer surface of the graphite electrode (FIGS.1D and 1E). The coating comprises electrochemically reduced grapheneoxide. In one embodiment, the coating consists of electrochemicallyreduced graphene oxide. In one embodiment, the modified graphiteelectrode has a distinct interface between the coating and the outersurface of the graphite electrode. In another embodiment, the modifiedgraphite electrode has no distinct interface between the coating and theouter surface of the graphite electrode. Rather, the coating isintegrated into or merged with the graphite substrate of the graphiteelectrode. Exemplary graphite electrodes include, but are not limitedto, glassy carbon electrode, pyrolytic carbon electrode, carbon pasteelectrode, and pencil lead electrode. Preferably, the graphite electrodeis a pencil lead electrode. The pencil lead is typically classified interms of hardness grades. Hardness grades are associated on a hardnessscale with the letter H or B or a combination of both, HB. Pencil leadof grades 9H, 6H, 4H, HB, B, or 8B may be used. A length of the pencillead may range from 20-70 mm, preferably 40-70 mm, more preferably 45-65mm. A diameter of the pencil lead may range from 0.13-1.40 mm,preferably 0.3-1 mm, more preferably 0.5-0.7 mm. The pencil lead may bemade from beneficiated graphite, milled graphite, intercalated graphite,a graphite intercalation compound, such as MC₈ (M=K, Rb and Cs) and M′C₆(M′=Li⁺, Sr²⁺, Ba²⁺, Eu²⁺, Yb³⁺, and Ca²⁺), graphite bisulfate,halogen-graphite compounds, and mixtures thereof. In a preferredembodiment, the graphite electrode is a mechanical pencil lead. In analternative embodiment, the graphite electrode may be a pencil lead, forexample, from a regular wooden or mechanical pencil. The wood casing maybe removed with a knife until the pencil has a diameter of 3-6 mm,preferably 4-6 mm, more preferably 4.5-5.5 mm. One end of the woodenpencil may be removed by the knife to expose a pencil lead of a lengthranging from 1-10 mm, preferably 1-5 mm, more preferably 2-3 mm. Thepencil lead may then be polished, for example, with sandpaper and/or apolishing pad. In one embodiment, the graphite electrode is not a glassycarbon electrode. The graphite electrode has an electroactive surfacearea ranging from 0.040-0.070 cm², preferably 0.045-0.060 cm², morepreferably 0.045-0.055 cm². The modified graphite electrode has anelectroactive surface area ranging from 0.3-0.6 cm², preferably 0.3-0.5cm², more preferably 0.3-0.4 cm². The electroactive surface area iscalculated by the Randles-Sevcik equation:I _(p)=2.69×10⁵ n ^(3/2) CAγ ^(1/2) D ^(1/2)Where I_(p) is the peak current, n is number of electrons participatingin the redox reaction, C is analyte concentration (mol L⁻¹), A is theelectroactive surface area (cm²), γ is the scan rate (V s⁻¹), and D isthe diffusion coefficient (cm²s⁻¹).

The modified graphite electrode may comprise 0.001-5 wt % of thecoating, preferably 0.001-2 wt %, preferably 0.01-1 wt %, morepreferably 0.05-1 wt %, based on a total weight of the modified graphiteelectrode. The coating may cover at least one end of the graphiteelectrode, and at least 50% of the outer surface area of the graphiteelectrode, preferably at least 70%, more preferably at least 90%. Athickness of the coating may range from 0.001-5 μm, preferably 0.001-1μm, more preferably 0.001-0.1 μm. The thickness of the coating may taperfrom a first end of the coating to a second end of the coating. Forexample, the first end of the coating is thicker, and the thickness mayrange from 0.009-5 μm, preferably 0.5-5 μm, more preferably 1-5 μm. Thesecond end of the coating is thinner, and the thickness may range from0.001-2 μm, preferably 0.001-0.5 μm, more preferably 0.001-0.05 μm. In apreferred embodiment, the coating has a uniform thickness. A surfaceroughness of the coating may range from 1-10 nm, preferably 1-5 nm, morepreferably 1-2 nm. A surface of the coating may comprise ridges. Theridges may cover up to 10% of the surface of the coating, preferably upto 30%, more preferably up to 60%. The ridges may be arranged in asubstantially parallel manner relative to each other. As used herein,the term “substantially parallel” refers to the vast majority of theridges are separated by substantially the same distance. For example, atleast 60%, preferably at least 80%, more preferably at least 90% of theridges may be aligned at no more than 10°, preferably no more than 7°,more preferably no more than 5°, relative to adjacent ridges. In someembodiments, the ridges have a shape shown in FIG. 1A. In otherembodiments, the ridges have a collapsed shape shown in FIG. 1B. In anembodiment, the ridges have a folded shape shown in FIG. 1C. A width ofthe ridges may range from less than 1 nm to 200 nm, preferably 5-150 nm,preferably 10-100 nm, more preferably about 25-75 nm. Additionally, aheight of the ridges (the distances from the base of the ridges to thetop of the ridges) may range from 0.5-10 nm, preferably 0.9-8 nm, morepreferably 2-6 nm. A length of a ridge may range from 0.5-40 μm,preferably 1-30 μm, more preferably 5-25 μm. A distance betweensuccessive ridges may range from 0.1-5 nm, preferably 0.1-3 nm, morepreferably 0.5-2 nm.

The coating may comprise pores which may be sized to hold electrolyteions, thus promoting diffusion of the electrolyte ions into the coatingand thereby improving the performance of the modified graphiteelectrode. A size of the pores may range from 0.1-10 nm, preferably0.1-5 nm, more preferably 0.1-2 nm. A porosity of the coating may rangefrom 0.1-99.9%, preferably 1-30%, more preferably 20-30%. The thicknessand the surface morphology of the coating may be studied by atomic forcemicroscopy, scanning electron microscopy, transmission electronmicroscopy, and scanning tunneling microscopy.

The electrochemically reduced graphene oxide may be in the form of aflake or a sheet, preferably a sheet with a structure similar to that ofgraphene. The sheets of electrochemically reduced graphene oxide may bearranged in a substantially planar manner relative to each other so asto form a layered structure with 2-100 layers, preferably 2-60 layers,more preferably 2-30 layers. As used herein, the term “substantiallyplanar” refers to the vast majority of the sheets within the coating aregenerally located within the same average plane or within substantiallyparallel planes. For example, at least 60%, preferably at least 80%,more preferably at least 90% of the electrochemically reduced grapheneoxide sheets may be aligned along a major axis at no more than 10°,preferably no more than 7°, more preferably no more than 5°, relative toadjacent sheets. The term “substantially planar” does not mean that theelectrochemically reduced graphene oxide sheets per se are flat becauseat a molecular level, the sheets may have a corrugated or undulatingconfiguration which leads to the aforementioned roughness of thecoating. An interlayer distance may vary from 0.1-10 nm, preferably0.5-5 nm, more preferably 0.5-2 nm.

In some embodiments, the graphene sheets covering the surface of thegraphene-modified graphite pencil working electrode have an area of 0.2mm² or greater, or 0.5 mm² or greater, or 0.8 mm² or greater, or 1 mm²or greater, or 1.5 mm² or greater, or 3 mm² or greater.

The reduced graphene oxide may be attracted to the outer surface of thegraphite electrode by covalent interactions such as van der Waals'forces and π-π interactions.

The graphene oxide may be bought, prepared with the Hummers' method, oran improved Hummers' method known to those skilled in the art (D. C.Marcano et al. ACS Nano 4 (2010) 4806-4814, incorporated herein byreference in its entirety). Graphite, such as flake graphite, isoxidized to give graphite oxide that comprises at least one functionalgroup with an oxygen atom such as a carbonyl group, a carboxyl group, ora hydroxyl group. In graphite oxide, the crystallinity of the graphiteis impaired and the distance between layers is increased. Therefore,graphene oxide can be easily obtained by separation of the graphiteoxide layers from each other by ultrasonication. An amount of oxygen ingraphene oxide may be at least 50 wt %, preferably 50-70 wt %, morepreferably 50-60 wt %, based on a total weight of graphene oxide.Graphene oxide may be reduced electrochemically thereby forming reducedgraphene oxide and/or partially reduced graphene oxide. An amount ofresidual oxygen in the partially reduced graphene oxide may be betweenat least 20 wt % and not more than 50 wt %, preferably 25-40 wt %, morepreferably 35-40 wt %, based on a total weight of the coating. An amountof residual oxygen in the reduced graphene oxide may be between at least2 wt % and not more than 20 wt %, preferably 5-15 wt %, more preferably5-10 wt %, based on the total weight of the coating. The amount ofresidual oxygen may be measured by X-ray photoelectron spectroscopy,energy-dispersive X-ray spectroscopy, and auger electron spectroscopy.

The modified graphite electrode may be utilized to detect analytes. Asused herein, the term “analyte” refers to a substance that is (or whosechemical constituents are) being identified, detected, and/or measuredby the modified graphite electrode. An analyte may be a component of afluid (e.g., vapor or liquid) sample in which the modified graphiteelectrode is immersed. Exemplary analytes include, without limitation,uric acid, tyrosine (N. Baig, A. Kawde Anal. Methods, 7 (2015)9535-9541, incorporated herein by reference in its entirety),biologically important catecholamines (tetrachlorohydroquinone, caffeicacid, rutin, p,p′-bisphenol, 3,4-dihydroxyphenylacetic acid,3,4-di-t-butylcatechol, hydroquinone, catechol, isoproterenol,3,4-dihydroxyephedrine, epinephrine, 3,4-dihydroxybenzylamine, dopamine,norepinephrine, 2,5-dihydroxybenzene-p-disulfonic acid), analytes ofenvironmental interest, such as picric acid, 2,4-dinitrophenol,plunavin, trifluralin, 4-amino-2-nitrophenol, p-nitrophenol,p-nitroaniline, alkylphenols (4-methylthiophenol, 4-methylthio-o-cresol,carbofuran phenol, 2,3,6-trimethylphenol, 2,4-dimethylphenol,2,3,5-trimethylphenol, 3,5-di-t-butylphenol, 4-methylphenol,2-methylphenol, 2-isopropylphenol, phenol, terbutalin, and3,5-dimethylphenol), and chlorophenols (2-benzyl-4-chlorophenol,2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol,4-chloro-3,5-dimethylphenol, pentachlorophenol, 2,4-dichlorophenol,2-chlorophenol, 4-chlorophenol, 2,4,5-trichlorophenol,2,5-dichlorophenol, and 3-chlorophenol), biologically important analytes(glucose, lactate, oxygen, glutamate, choline, phosphate, acetylcholine,dioxybutyrate, homocysteine, D-cysteine, creatine, creatinine, sucrose,fructose, nitric oxide, galactose, arsenite, cholesterol, fructosamine,bilirubin, glycine, methionine, L-citrulline, phosphatidic acid,lysophosphatidic acid, arachidonic acid, asymmetric dimethylarginine,1,3-diaminopropane, 21-deoxycortisol, aminoadipic acid,D-2-hydroxyglutaric acid, L-2-hydroxyglutaric acid, aminoadipic acid,2-hydroxyadipic acid, oxoadipic acid, oxoglutaric acid,7-hydroxyprogesterone, 3-hydroxyisovaleric acid, 3-hydroxymethylglutaricacid, 3-methylcrotonylglycine, 3-methylglutaconic acid, adipic acid,ammonia, methylglutaric acid, (S)-3-hydroxyisobutyric acid,3-hydroxyisovaleric acid, 3-methylcrotonylglycine, 3-hydroxyisovalericacid, pyruvic acid, (S)-3,4-dihydroxybutyric acid, pyroglutamic acid,ganglioside GM3, glucosylceramide, lactosylceramide,tetrahexosylceramide, trihexosylceramide, 2-hydroxyestradiol,2-hydroxyestrone, 20-hydroxyeicosatetraenoic acid,5-acetylamino-6-amino-3-methyluracil, alpha-N-phenylacetyl-L-glutamine,androstenedione, benzoic acid, bromide, cadaverine, cholic acid,coproporphyrin I, coproporphyrin III, deoxycholic acid, deoxycytidine,DHEA sulfate, DL-homocystine, estradiol, estriol, estrone, estronesulfate, fluorine, glycocholic acid, guanine, hexanalhydroxyphenyllactic acid, iodide, L-aspartic acid, L-cysteine,L-glutamine, L-lactic acid, L-malic acid, L-methionine, malondialdehyde,myoinositol hexakisphosphate, N-acetylaspartylglutamic acid, orotidine,progesterone, salicyluric acid, selenomethionine, thymine, uric acid,vanilpyruvic acid, cortisol, anabasine, cotinine, hydroxycotinine,L(−)-nicotine pestanal nornicotine, heptacarboxylporphyrin I, enkephalinL, 24-hydroxycholesterol, 27-hydroxycholesterol, deoxyadenosine,1-methyladenine, succinyladenosine, hexacosanoic acid, phytanic acid,pristanic acid, L-pipecolic acid, erucic acid, 7C-aglycone, 5C-aglycone,(R)-salsolinol, alpha-carotene, 5-methyltetrahydrofolic acid, butyricacid, mannitol meopterin, quinolinic acid, 2-butanol, acetone, butanone,ethanol, isopropyl alcohol, methanol, acetaldehyde, nicotinic acid,pantothenic acid, riboflavin, scyllitol, thiamine, honmogentisic acid,aminoadipic acid, L-histidine, 1,5-anhydrosorbitol, 1-methylhistidine,3,4-dihydroxybenzeneacetic acid, 3-methylhistidine, 4-hydroxy-L-proline,4-hydroxynonenal, 5-hydroxylysine, 8-hydroxyguanine, 8-hydroxyguanosine,anscrine, carnosine, citrulline, epsilon-(ganlilla-glutamyl)-lysine,folic acid, fumaric acid, galactitol, ganlilla-aminobutyric acid,glycerophosphocho line, glycylproline, hydroxyproline,L-2,4-diaminobutyric acid, L-alpha-aminobutyric acid, L-arabitol,L-arginine, L-asparagine, L-cystathionine, L-DOPA, L-glutamic acid,L-isoleucine, L-leucine, L-lysine, L-phenylalanine, L-proline, L-serine,L-threonine, L-tryptophan, L-valine, methylmalonic acid, myoinositol,ornithine, pentosidine, phosphorylcholine, prolylhydroxyproline,ribitol, sorbitol, succinic acid, thiamine monophosphate, thiaminepyrophosphate, estriol-3-sulfate-16-glucuronide, estriol-3-glucuronide,acetylglycine, N-acetylserine, L-thyronine, prostaglandin E2, kynurenicacid, 24,25-dihydroxyvitamin D, 25,26-dihydroxyvitamin D,25-hydroxyvitamin D2, calcidiol, ergocalciferol, vitamin D3,11-dehydro-thromboxane B2, 5a-tetrahydrocortisol, ethylmalonic acid,FAD, flavin mononucleotide, glutaric acid, isovalerylglycine,liothyronine, suberic acid, tetrahydrocortisone, thyroxine,3-hydroxybutyric acid, acetoacetic acid, isocitric acid, L-glutamicacid, L-malic acid, oxalacetic acid, indolcacetic acid, argininosuccinicacid, uracil, 3-methoxytyrosine, 5-mydroxyindoleacetic acid,homovanillic acid, N-acetyl-L-tyrosine, N-acetylvanilalanine,vanillylmandelic acid, vanylglycol, taurocyamine, aspartylglycosamine,1,3,7-trimethyluric acid, 1,3-dimethyluric acid, 1,7-dimethyluric acid,1-methylxanthine, 11 b-PGF2a, 3-chlorotyrosine, 3-methylxanfhine,5-HETE, 7-methylxanthine, caffeine, paraxanthine, theobromine,theophylline, iodotyrosine, dimethyl-L-arginine,13S-hydroxyoctadecadienoic acid, symmetric dimethyl arginine,androstanediol, trans-trans-muconic acid, 2-methyl-3-hydroxybutyricacid, 2-methylacetoacetic acid, tiglylglycine, acetaminophenglucuronide, ubiquinol, dihydrothymine, urcidoisobutyric acid,chenodeoxycholic acid, chenodeoxycholic acid glycine conjugate,hyaluronic acid, taurochenodesoxycholic acid, taurocholic acid,1b,3a,12a-trihydroxy-5b-cholanoic acid, hyocholic acid, hyodeoxycholicacid, ursodeoxycholic acid, isoursodeoxycholic acid, lithocholic acid,ursocholic acid, 2-methylcitric acid, 3-methyl-crotonylglycine,hydroxypropionic acid, 2-pyrrolidinone, dimethyl amine, 8-isoprostane,ascorbic acid, glutathione, o-phosphoethanolamine, 3,5-diiodothyronine,1,3-diaminopropane, 1-methylguanosine, 16a-hydroxyestrone, enterodiol,enterolactone, N1-acetylspermidine, N8-acetyl-spermidine, perillic acid,perillyl alcohol, ribothymidine, xanthosine, testosterone, 1-methyluricacid, 3-methyladenine, citric acid, cytidine, hypoxanthine, inosine,N-acetyl-L-aspartic acid, orotic acid, oxidized glutathione,pseudouridine, thymidine, uridine, xanthine, 1-methylinosine,16a-hydroxydehydroisoandrosterone, 5a-tetrahydrocorticosterone,alpha-linolenic acid, alpha-tocopherol, B-carotene, beta-cortol,docosahexaenoic acid, docosapentaenoic acid, gama-tocopherol, linoleicacid, lycopene, putrescine, tetrahydrodeoxycorticosterone,tetrahydrodeoxycortisol, vitamin A, L-fucose, prostaglandin F2a,leukotriene B4, 6-ketoprostaglandin Fla, sebacic acid, butyrylcarnitine,decanoylcamitine, dodecanoylcamitine, isovalerylcarnitine,L-hexanoylcamitine, L-octanoylcarnitine, L-palmitoylcarnitine,lactulose, propionylcarnitine, stearoylcarnitine, tiglylcarnitine,dihydrouracil, 5-alpha-cholestanol, lathosterol, 1-methyladenosine,3,5-diiodo-L-tyrosine, betaine, cyclic AMP, guanidine, guanidinosuccinicacid, guanidoacetic acid, methyl guanidine, picolinic acid,2,3-butanediol, 2-hydroxyphenethylamine, 2-oxoarginine,4-guanidinobutanoic acid, 7a-hydroxycholesterol, argininic acid,cholesterol sulfate, homo-L-arginine, methanethiol, p-octopamine,propylene glycol, sulfolithocholylglycine, tyramine, urea, L-kynurenine,beta-leucine, cob(I)alamin, inosinic acid, 16-a-hydroxypregnenolone,pyridinoline, histamine, lipoxin A4, hydrogen peroxide, thromboxane A2,D-xylose, 19-hydroxyandrost-4-ene-3,17-dione, glyceric acid,L-a-glutamyl-L-lysine, corticosterone, cortisone, 1-methylhistamine,(R)-3-hydroxybutyric acid, (R)-3-hydroxyisobutyric acid,(S)-3-hydroxyisobutyric acid, 1-butanol, 4-heptanone, D-Lactic acid,glycerol, hyaluronan, L-carnitine, pyruvaldehyde, S-adenosylmethionine,hydrogen carbonate, ureidopropionic acid, beta-alanine, cortol,cortolone, leukotriene C4, leukotriene E4, adenosine triphosphate, ADP,guanosine diphosphate, guanosine triphosphate, p-hydroxyphenylaceticacid, taurine, 2-methylbutyrylglycine, isobutyrylglycine, methylsuccinicacid, N-butyrylglycine, epitestosterone, thyroxine sulfate,etiocholanolone, diphenhydramine, 3-hydroxydodecanoic acid, diadenosinehexaphosphate, diadenosine pentaphosphate, diadenosine tetraphosphate,diadenosine triphosphate, xanthurenic acid, cyanocobalamin, pyridoxine,hydrogen sulfide, thiosulfate, aldosterone 18-glucuronide, p-synephrine,m-tyramine, serotonin, 1-naphthol, 2-naphthol, retinyl ester,2-pyrocatechuic acid, gentisic acid, dopamine glucuronide, isomaltose,melanin, N2,N2-dimethylguanosine, phenylacetic acid, trimethylamineN-oxide), and mixtures thereof.

Preferably, the analyte is uric acid. In the human body, uric acid isproduced by the oxidation of purine (R. Goyal, V. Gupta, A. Sangal, N.Bachheti, Electroanalysis. 17 (2005) 2217-2223, incorporated herein byreference in its entirety). It is an important biomarker for certaindiseases. Uric acid is present in the urine, serum, and blood. It ismainly excreted from the body with the help of the kidney. An abnormallevel of uric acid in the body is responsible for a number of diseasessuch as Lesch/Nyhan syndrome and hyperuricemia, which could cause gout(R. Goyal, V. Gupta, A. Sangal, N. Bachheti, Electroanalysis. 17 (2005)2217-2223, incorporated herein by reference in its entirety). A highconcentration of the uric acid causes cardiovascular disease and kidneydamage (R. Goyal, V. Gupta, A. Sangal, N. Bachheti, Electroanalysis. 17(2005) 2217-2223, incorporated herein by reference in its entirety). Therisk of diabetes mellitus is high in patients with a high concentrationof uric acid (A. Costa, I. Iguala, J. Bedini, L. Quinto, I. Conget,Metabolism. 51 (2002) 372-375, incorporated herein by reference in itsentirety). Therefore, the quantification of uric acid is highlyimportant. A major problem in the electrochemical quantification of uricacid is the interference of the ascorbic acid, which has an oxidationpotential similar to that of uric acid (J. He, G. Jin, Q. Chen, Y. Wang,Anal. Chim. Acta. 585 (2007) 337-343, incorporated herein by referencein its entirety).

Therefore, the second aspect of the disclosure relates to a method ofdetecting uric acid, comprising immersing the modified graphiteelectrode, a reference electrode, and a counter electrode in an aqueoussample comprising uric acid, and applying a square wave potential to theaqueous sample. A concentration of the uric acid may fall within alinear response curve of the modified graphite electrode and may rangefrom 0.02-1,000 μM, preferably 0.2-0.5 μM, more preferably 0.2-0.22 μM.

The reference electrode may be a standard hydrogen electrode, a normalhydrogen electrode, a reversible hydrogen electrode, a saturated calomelelectrode, a silver chloride (Ag/AgCl) electrode, or a dynamic hydrogenelectrode. Preferably, the reference electrode is a silver chlorideelectrode. The counter electrode may comprise platinum, gold, or carbon.Preferably, the counter electrode is a platinum wire.

Preferably, the at least one end of the modified graphite electrodecoated with electrochemically reduced graphene oxide may be in contactwith the aqueous sample. The modified graphite electrode is inelectrical communication with the reference electrode. A potential isapplied between the reference electrode and the modified graphiteelectrode to produce a current. Changes in the current as a result ofreduction/oxidation/decomposition of detected analytes (e.g. uric acid)can be used to determine the amount of analyte in the sample into whichthe electrode is placed. The biasing potential may have the waveform ofa linear sweep voltammetry, square wave voltammetry, or a cyclicvoltammetry. Preferably, square wave voltammetry is used, and thewaveform may have an amplitude ranging from 0.01-0.1 V, preferably0.01-0.05 V, more preferably 0.01-0.03 V. A frequency of the square wavevoltammetry waveform may range from 10-100 Hz, preferably 30-60 Hz, morepreferably 45-60 Hz. An adsorption time ranges from 10-240 s, preferably50-180 s, more preferably 90-160 s. The modified graphite electrode maybe reusable and may be capable of repeated detection without calibrationor replacement.

The presence of biomolecules and common ions mostly does not interferewith the detection of uric acid in the solution using the disclosedmethod. Exemplary biomolecules include, without limitation,phenylalanine, alanine, glucose, fructose, L-methionine, and ascorbicacid. Exemplary common ions include, without limitation, Na⁺, K⁺, Li⁺,Ni²⁺, SO₄ ²⁻, and Cl⁻. Because of no or low interference from othermolecules, the method can be used to detect uric acid in various samplessuch as whole blood, plasma, serum, saliva, sweat, urine, washes oftissues, extracts of tissues, amniotic fluid, and placental fluid.

The third aspect of the disclosure relates to a sensing device. Themodified graphite electrode may be part of (e.g. integrated in) thesensing device which comprises the aforementioned reference electrodeand counter electrode. The sensing device may include a housing thatcomprises at least one modified graphite electrode, and a fluiddistribution manifold that comprises a fluid flow path that is in fluidcommunication with the modified graphite electrode, the counterelectrode, and the reference electrode. The fluid flow path can bring afluid comprising at least one analyte in contact with the modifiedgraphite electrode for sensing.

The sensing device may be in communication with at least one readoutdevice that may generally be capable of measuring the current and/orpotential at the modified graphite electrode. In most embodiments, thereadout device may be a set of electronics. An electronic readoutdevice, for example, may be capable of detecting current changes.Moreover, the readout device may be a component of the sensing device ormay be separated from the sensing device. Furthermore, the readoutdevice may also be linked to an adapter that can interface with acontroller device. Preferably, a readout circuit used to enabledetermination of the presence and/or amount of analyte may form part ofthe readout device. In some embodiments, the readout circuit may beconfigured to measure the current and/or the potential at the modifiedgraphite electrode. The readout circuit may also be configured toindicate the current and/or potential value(s) to a user of the sensingdevice such that he/she can detect the presence of the analyte andquantify it based on this measurement. To achieve this, the readoutcircuit may comprise an electronic display and/or a loudspeaker forpresenting the current and/or potential value(s) to the user, and mayfurther comprise a transmitter (or transceiver) for transmitting thedata to another device. The latter feature enables the user to monitorthe environment from a remote location. In another embodiment, thereadout circuit may be configured to determine the presence and/oramount of analyte using the current and/or potential value(s) andindicate the result to the user (with or without the current and/orpotential value(s)). This embodiment therefore provides the user withthe end result without requiring him/her to derive it from the raw data.

In practice, this analysis would be performed by the processor incombination with a storage medium. For example, a processor may beconfigured to receive the current and/or potential value(s) from thereadout circuit and compare this with predetermined calibration data(e.g. predetermined measurements of current and/or potential differenceversus analyte concentration) from the storage medium to determine thepresence and/or amount of analyte.

The fourth aspect of the disclosure relates to a method of making themodified graphite electrode comprising: (i) dispersing graphene oxide ina buffer to form a graphene oxide dispersion, (ii) immersing a graphiteelectrode, a reference electrode, and a counter electrode in thegraphene oxide dispersion, and (iii) applying a cyclic potential to thegraphene oxide dispersion to coat an outer surface of the graphiteelectrode with a coating comprising electrochemically reduced grapheneoxide thereby forming the modified graphite electrode.

The graphene oxide dispersion may be prepared by dispersing grapheneoxide in the buffer by sonicating for 0.5-5 hours, preferably 1-4 hours,more preferably 1-3 hours. A concentration of the graphene oxide in thegraphene oxide dispersion ranges from 1.0-5.0 mg/ml, preferably 2.0-4.0mg/ml, more preferably 2.5-3.5 mg/ml. The buffer may be a citratebuffer, a phthalate buffer, or an acetate buffer, preferably an acetatebuffer. A concentration of the buffer may range from 0.01-0.50 M,preferably 0.05-0.20 M, more preferably 0.05-0.15 M. A pH of the buffermay range from 4.0-5.0, preferably 4.5-5.0, more preferably 4.7-4.9.

The graphite electrode may be polished before it is immersed into thegraphene oxide dispersion. For example, the graphite electrode can bepolished with alumina particles with a size ranging from 0.05-0.5 μm,preferably 0.1-0.5 μm, more preferably 0.2-0.3 μm. The outer surface ofthe graphite electrode may be rinsed with solvents, such as ethanol,acetone, and water, to remove impurities. A length of the graphiteelectrode immersed in the graphene oxide dispersion may range from 20-70mm, preferably 40-70 mm, more preferably 45-50 mm.

The cyclic potential may range from −1.6 V to 0.6 V, preferably −1.5 Vto 0.4 V, more preferably −1.4 V to 0.3 V. The scan rate may range from10-120 mV/s, preferably 50-120 mV/s, more preferably 90-110 mV/s. Anumber of cycles may vary from 3-10, preferably 4-10, more preferably4-6. The whole process may take up to 60 minutes, preferably up to 40minutes, more preferably up to 30 minutes.

The present embodiments are being described with reference to specificexample embodiments and are included to illustrate but not limit thescope of the invention.

Example 1 Materials and Methods

Uric acid, ascorbic acid, L-methionine, glucose, fructose, sodiumchloride and potassium chloride were obtained from Sigma-Aldrich (USA).Sodium phosphate monobasic and di-potassium hydrogen orthophosphate wasreceived from BDH (England). Alanine and L-phenylalanine were obtainedfrom Fluka (USA). Graphite was received from Fischer Science Education(USA). Double distilled water was used for solution preparation andexperimental work. The double distilled water was collected directlyfrom Water Still Aquatron A 4000 D (UK).

Square wave voltammetry, cyclic voltammetry, and electrochemicalimpedance spectroscopy experiments were performed using athree-electrode system electrochemical workstation (Auto Lab,Netherland). The working electrode was the bare graphite pencilelectrode (GPE) or the modified graphite electrode (dERGO-GPE), thecounter electrode was a platinum wire, and Ag/AgCl was used as referenceelectrode. The GPE, reference and counter electrodes were fixedvertically, and about 7 mm of the GPE was dipped into the analyticalsolution. The GPE was previously described in detail (A. Kawde, M. Aziz,N. Baig, Y. Temerk, J. Electroanal. Chem. 740 (2015) 68-74, incorporatedherein by reference in its entirety). All experiments were conducted ina 3-ml glass cell. GR-2000 electrical balance was used for themeasurements of all weights. Accumet® XL50 pH meter was used for thecontrol of the buffers pH. FE-SEM images of the bare GPE and dERGO-GPEwere recorded using TESCAN LYRA 3 (Brno, Czech Republic) at the Centerof Research Excellence in Nanotechnology, KFUPM. Raman and FTIR spectraof the graphite and graphene oxide were collected by HORIBA ScientificLabRAM HR Evolution and NICOLET 6700 FT-IR, respectively.

Example 2 Characterization of Graphene Oxide (GO)

GO was prepared by the Hummers method. The synthesized GO wascharacterized by the FT-IR. The FT-IR spectrum (FIG. 2) indicated thepresence of hydroxyl group (ν=3425 cm⁻¹), carboxyl carbonyl (ν=1733cm⁻¹), aromatic carbon-carbon double bond (ν=1625 cm⁻), carboxylic acid—C—O stretching (ν=1383 cm⁻¹), epoxy —C—O stretching (ν=1225 cm⁻¹), andalkoxy —C—O stretching (ν=1050 cm⁻) (T. Yang, L. Liu, J. Liu, M.-L.Chen, J.-H. Wang, J. Mater. Chem. 22 (2012) 21909-21916, incorporatedherein by reference in its entirety). The presence of these entirefunctional groups confirmed the formation of the dark brown grapheneoxide (FIG. 6) from graphite (FIG. 5). The FT-IR spectra of graphite(FIG. 1F) did not show any significant peak. FIGS. 3 and 4 are the Ramanspectra of graphite and synthesized graphene oxide, respectively. A weakD, 2D, and strong G-band was observed in the FIG. 3. On the other hand,graphene oxide Raman spectra (FIG. 4) revealed a prominent D and G bandappeared at 1350 cm⁻¹ and 1594 cm⁻¹, respectively. A relative shift in Dand G band of the graphene oxide was observed compared to the graphite(A. Alhwaige, T. Agag, H. Ishida, S. Qutubuddin, RSC Adv. 3 (2013)16011-16020, incorporated herein by reference in its entirety).

Example 3 Preparation of dERGO-GPE

The parameters affecting the preparation of dERGO-GPE were studied andadvantageous conditions were listed in Table 1. A 3 mg/ml graphene oxidein 0.1 M acetate buffer (pH 4.8) solution was prepared and sonicated for2 hours to obtain a uniform and stable dispersion. The dispersedgraphene oxide was transferred into the 3-ml cell, and the GPE,reference and counter electrodes were immersed into the graphene oxidedispersion. The graphene oxide directly reduced onto the GPE surfacewhen a cyclic potential swept at a scan rate of 20 mV/s from −1.4 V to0.3 V over 5 cycles. The modified electrode was washed gently by dippingin the double distilled water prior to analysis.

The morphology of the reduced GO was studied by FE-SEM. The presence ofgraphene layers was observed on the surface of the modified GPE (FIG. 8)compared to the bare GPE (FIG. 7) where no such layers are present. Thepresence of layers indicated GO has successfully reducedelectrochemically on the GPE surface. Moreover, from FIG. 8, it could beobserved the graphene sheets are wrinkled. The wrinkled graphene sheetsnot only provide the stability to not easily revert to the graphiticform and also increased the surface area (R. Kou, Y. Shao, D. Wang, M.Engelhard, J. Kwak, J. Wang, V. Viswanathan, C. Wang, Y. Lin, Y. Wang,I. Aksay, J. Liu, Electrochem. Commun. 11 (2009) 954-957, incorporatedherein by reference in its entirety).

TABLE 1 Conditions for formation of dERGO-GPE (nos. 1-4) and analysis ofuric acid (nos. 5 and 6) Sr # Parameters Best response 1 Scan rate forgraphene reduction 0.1 V/s 2 Graphene concentration 3 mg/mL 3 Scanwindow for graphene oxide reduction −1.4 V to 0.3 V 4 Scan number forgraphene oxide reduction 5 5 Electrolyte 0.1M PBS 6 Technique SWV

The parameters affecting the analysis of uric acid were studied, and itwas found that an advantageous response was obtained with 0.1 M PBS andsquare wave voltammetry (SWV).

Example 4 Study of the Electroactive Surface Area and ElectrochemicalBehavior of dERGO-GPE

In order to elucidate the electroactive behavior of the graphene on theGPE surface, the electroactive surface area of the dERGO-GPE and bareGPE was calculated by the Randles-Sevcik equation:I _(p)=2.69×10⁵ n ^(3/2) CAγ ^(1/2) D ^(1/2),  (1)

where n is electrons number participate in the redox reaction, C isanalyte concentration in molL⁻, A is electrode electroactive surfacearea in cm², γ is the scan rate in Vs⁻ and D represents diffusioncoefficient in cm²s⁻¹. FIGS. 10 and 12 show the direct relation of peakcurrent (I_(p)) and the square root of scan rate (γ^(1/2)). All otherparameters n, C, and D in the Randles-Sevcik equation are constant. Theelectroactive surface area was calculated from equation 1 by sweepingthe potential at a different rate from 20 mV/s to 100 mV/s in a solutioncontaining 0.1 M KCl and 5 mM K₃Fe(CN)₆/K₄Fe(CN)₆. The electroactivesurface area of bare GPE and dERGO-GPE was 0.0686 cm² and 0.3615 cm²,respectively. This showed the graphene immensely affect theelectroactive surface area of the GPE.

The interface properties of the bare and dERGO-GPE were analyzed byusing the electrochemical impedance spectroscopy, a 5 mV potential inthe frequency range of 100 kHz to 0.01 Hz was applied to the workingelectrode. FIGS. 13 and 14 describe the electrochemical spectra of bareGPE (FIG. 13) and dERGO-GPE (FIG. 14) in a solution containing 5 mMK₃Fe(CN)₆/K₄Fe(CN)₆ and 0.1M KCl. The frequency for the electrochemicalimpedance spectra swept from 100 kHz and 0.01 Hz. The two axes −Z and Zindicate the imaginary and real value of the impedance variables,respectively. Nyquist plot consists of two parts: one is semicircle partand other is the straight line. The charge transfer resistance (R_(ct))for bare GPE was 2954Ω and almost a straight line was observed for thedERGO-GPE. R_(ct) values determined from the semicircle part andstraight line of the impedance spectra correspond to the diffusioncontrol process. The R_(ct) value of the modified electrode was muchless than the unmodified electrode. The low value of the R_(ct) revealedthe surface of the dERGO-GPE became more conductive due to the presenceof graphene layers compared to the bare GPE.

FIGS. 15 and 16 compare the behavior of the bare GPE (FIG. 15) anddERGO-GPE (FIG. 16) for 1 mM uric acid in 0.1 M PBS buffer (pH 7.0) bycyclic voltammetry. A prominent sharp peak was observed on thevoltammogram at dERGO-GPE whereas the voltammogram at GPE showed a broadpeak. A significant increase in current was obtained with dERGO-GPEcompared to the bare GPE. The uric acid oxidation peak on the bare andmodified GPE appeared at 0.40 and 0.35 V, respectively.

The electrode surface plays a role in the reversibility and the kineticsof the electrochemical reaction. The cyclic voltammograms reveal thereversibility of the electrochemical reaction (T. Ndlovu, O. Arotiba, S.Sampath, R. Krause, B. Mamba, Int. J. Electrochem. Sci. 7 (2012)9441-9453, incorporated herein by reference in its entirety). Equation 2was used to calculate the potential difference between the anodic peakpotential (E_(pa)) and the cathodic peak potential (E_(pc)) of thecyclic voltammograms of uric acid in 0.1 M PBS.ΔE=E _(pa) −E _(pc)=59/n  (2)

In the case of irreversible reaction, only a single peak appears. Thereaction is considered as reversible when the value of ΔE is 59/n mV andquasi-reversible if the value is greater than 59/n mV. The ΔE valuecalculated for the uric acid was 37 mV. The value was higher compared tothe theoretical value 59/n mV which indicated the electrochemicalreaction on the electrode surface was quasi-reversible. The cyclicvoltammogram of bare electrode revealed the electrochemical reaction ofthe uric acid on the surface was irreversible, and no reduction peak wasobserved on bare GPE. This observation means that the reversibility ofreduction/oxidation of the uric acid was feasible on the modifiedelectrode surface. Moreover, the value of n calculated from equation 2was 1.6. This value revealed the two electrons contributing to theelectrochemical reaction of uric acid.

Example 5 Effect of pH on the Oxidation Peak of Uric Acid at dERGO-GPE

The supporting electrolyte pH has a significant effect on the peak shiftand also on peak current of the uric acid. The pH effect was scannedfrom 6.0 to 8.0 in a 0.1 M PBS buffer comprising 10 μM uric acid (FIG.17). The oxidation peak current slightly increased as the pH increasedand reached a maximum value at pH 7.0 and started decreasing after that.A linear negative shift in oxidation peak potential was observed as thepH is increased from the 6.0 to 8.0 (FIG. 19). Equation (3) representsthe equation of the line shown in FIG. 19. The slope of the linearrelation (R²=0.991) between pH and uric acid oxidation peak potentialwas −64.5 mV and is very close to the theoretical value −59 mv. Itindicated an equal number of protons and electrons were involved in theelectro-oxidation of the uric acid, which is in agreement with reportedwork (F. Zhang, Z. Wang, Y. Zhang, Z. Zheng, C. Wang, Y. Du, W. Ye,Talanta. 93 (2012) 320-325, incorporated herein by reference in itsentirety).E vs. Ag/AgCl=760.3−64.5 [pH]  (3)

Example 6 Investigation of Square Wave Voltammetry (SWV) Parameters

All the possible parameters of the square wave voltammetry that couldaffect the electrochemical oxidation of the uric acid on the dERGO-GPEsurface were studied. The amplitude was first varied between 0.020 V and0.060 V. The oxidation peak current reached the maximum at 0.03 V andthen continuously dropped with further increase of the amplitude (FIG.20). The frequency also had a significant influence on the oxidationsignal of the uric acid when the frequency was varied from 15 to 70 Hz.The maximum response was obtained at 50 Hz (FIG. 21). Finally, theadsorption time was investigated. The adsorption time has a significanteffect on the strength of the oxidation signal. The current increasedrapidly as the adsorption time increased and leveled off at 120 s and nofurther increase in current observed with time adsorption increase (FIG.22). This enormous increase in current revealed the graphene layer onGPE surface, significantly enhanced dERGO-GPE capability for uric acidadsorption.

Example 7 Calibration Curve and Detection Limit

The oxidation of uric acid was performed with the dERGO-GPE at theseconditions for SWV: 0.03 V amplitude, 50 Hz frequency and 120 sadsorption time (FIG. 23). The linear relationship was observed between0.2 μM and 22 μM uric acid with a regression constant of 0.996. (FIG.24) A linear equation yielded by the calibration curve is: I (μA)=136.08C_(UA)(μM) − 29.914. For dERGO-GPE, the limit of quantification was 0.2μM and the limit of detection was 0.037 μM which is better than most ofthe reported electrodes modified to contain graphene (Table 2). In thesecases, the glassy carbon electrode (GCE) is modified to contain grapheneor graphene composite. These methods are time-consuming due to thenumber of steps involved in the modification of the electrode. X. Wanget al. fabricated a palladium nanoparticle/graphene/chitosan modifiedGCE for uric acid detection with a limit of detection 0.17 μM and thepreparation of the electrode took 24 hours (X. Wang, M. Wu, W. Tang, Y.Zhu, L. Wang, Q. Wang, P. He, J. Electroanal. Chem. 695 (2013) 10-16,incorporated herein by reference in its entirety). Y. Xue et al.introduced a poly(diallyl dimethyl ammonium chloride)-graphenenanosheets modified GCE with a detection limit 0.1 μM. This modifiedelectrode is prepared by a time-consuming process has several steps (Y.Xue, H. Zhao, Z. Wu, X. Li, Y. He, Z. Yuan, Biosens. Bioelectron. 29(2011) 102-108, incorporated herein by reference in its entirety). Z.Zhang and J. Yin prepared a sensitive partially electro-reduced grapheneoxide modified GCE with a detection limit of 0.05 μM and took 12 hoursof preparation (Z. Zhang, J. Yin, Electrochim. Acta. 119 (2014) 32-37,incorporated herein by reference in its entirety). These modified GCEare prepared by the same method of casting of the graphenedispersion/solution onto the surface of the glassy carbon electrode andthen dried at room temperature (Y. Li, G. Ran, W. J. Yi, H. Q. Luo, N.B. Li, Microchim. Acta. 178 (2012) 115-121, incorporated herein byreference in its entirety). A major issue with the casting method isthat it is hard to control the thickness of the graphene layer. Inaddition, for all these references, several steps are involved in thepreparation of the graphene composite which was then cast on the surfaceof the GCE.

In this work, the control of the coating thickness is easier becausegraphene oxide is reduced directly onto the surface of GPE. In addition,the dERGO-GPE has a lower limit of detection (0.037 μM) than thereported modified GCE indicated that graphene oxide is more efficientlyreduced on the GPE surface.

TABLE 2 Comparison of the dERGO-GPE with other graphene modifiedelectrodes Electrode modification LOQ LOD Electrode Technique time (h)(μM) (μM) Ref. Screen-printed graphene electrode DPV 1.16 0.8 0.2 1 PdNPs/graphene/chitosan/GCE DPV 24 0.5 0.17 2 Au NPs-β-cyclodextringraphene/GCE SWV 1.2 0.5 0.21 3 Graphene Sheet-PTCA/GCE DPV C* 4 0.92 4Neutral red-functionalized graphene Amperometry C* 0.12 0.062 5nanosheets/GCE partially electro-reduced graphene SWV 12 0.1 0.05 6oxide/GCE Graphene poly(acridine red)/GCE DPV C* 0.8 0.3 7Pd₃Pt₁/PDDA-Reduced Graphene DPV C* 4 0.1 8 Oxide/GCE PDDA-Graphenenanosheets/GCE DPV — 0.5 0.1 9 Directly electrochemically reduced SWV0.46 0.2 0.037 This graphene oxide/GPE work DPV = differential pulsevoltammetry SWV = square wave voltammetry C* = certain amount of thegraphene oxide composite cast on the surface of the GCE and dried buttime is not mentioned. 1. J. Ping, J. Wu, Y. Wang, Y. Ying, Biosens.Bioelectron. 34 (2012) 70-76 2. X. Wang, M. Wu, W. Tang, Y. Zhu, L.Wang, Q. Wang, P. He, J. Electroanal. Chem. 695 (2013) 10-16 3. X. Tian,C. Cheng, H. Yuan, J. Du, D. Xiao, S. Xie, M. Choi, Talanta. 93 (2012)79-85 4. W. Zhang, Y. Chai, R. Yuan, S. Chen, J. Han, D. Yuan, Anal.Chim. Acta. 756 (2012) 7-12 5. J. Song, J. Qiao, S. Shuang, Y. Guo, C.Dong, J. Mater. Chem. 22 (2012) 602-608 6. Z. Zhang, J. Yin,Electrochim. Acta. 119 (2014) 32-37 7. Y. Li, G. Ran, W. J. Yi, H. Luo,N. Li, Microchim. Acta. 178 (2012) 115-121 8. J. Yan, S. Liu, Z. Zhang,G. He, P. Zhou, H. Liang, L. Tian, X. Zhou, H. Jiang, Colloids Surf. B.Biointerfaces. 111 (2013) 392-397 9. Y. Xue, H. Zhao, Z. Wu, X. Li, Y.He, Z. Yuan, Biosens. Bioelectron. 29 (2011) 102-108 References 1-9 areeach incorporated herein by reference in its entirety.

Example 8 Application and Interferences

The dERGO-GPE sensor was applied to a urine sample collected from ahealthy person and diluted with 0.1 M PBS. A sharp peak was observed at0.327 V. It was confirmed by spiking of 8, 10, 12 μM of uric and thepeak current increased linearly. The recoveries of the spiked uric acidlie between 98.2 and 105% (Table 3). The real sample has a number ofions, proteins and interfering species: the good recoveries indicatedthe developed electrode is foul free and could cope with real sampleinterferences. Ascorbic acid is the most commonly observed interferingspecies in the quantification of uric acid. It has been observed underconditions of SWV (25 Hz frequency, 0.02 V amplitude, and 30 sadsorption time), a prominent well-defined peak of the 200 μM ascorbicacid was observed. The preferred conditions for the detection of uricacid did not facilitate the oxidation of the ascorbic acid, and a smallpeak of ascorbic acid was observed. Although ascorbic acid is 50 timesmore concentrated than uric acid, there was just a 6% variation in theoxidation peak current for uric acid. Moreover, other interferences,such as 50 μM L-methionine, 50 μM L-alanine, 20 μM L-phenylalanine, 10μM fructose, and 10 μM glucose were also studied with 10 μM of uricacid. The current variation observed varied from 5 to 12%. This smallvariation of current indicated, the dERGO-GPE has the capability tobehave well in the presence of interfering species and remain foul freeeven at a much higher concentration of the ascorbic acid. Theinterfering studied indicated the interfering species and the highconcentration of ascorbic acid do not affect the sensitivity of theelectrode for uric acid. Therefore, the dERGO-GPE is a valuable tool forthe sensitive detection of the uric acid in the urine due to low cost,fast, good linear range, high sensitivity and low limit of detection.

TABLE 3 Recovery of spiked uric acid in the urine sample Sr # Found (μM)Added (μM) Recovered (μM) Percent recovery 1 5.15 8 7.90 98.75 2 5.15 109.82 98.2 3 5.15 12 12.71 105

The invention claimed is:
 1. A modified graphite electrode, comprising:a graphite electrode with an outer surface; and a coating on the outersurface of the graphite electrode, wherein the coating consists ofelectrochemically reduced graphene oxide, and the modified graphiteelectrode has an electroactive surface area in a range of 0.3-0.6 cm².2. The modified graphite electrode of claim 1, wherein the graphiteelectrode is in the form of a pencil lead.
 3. The modified graphiteelectrode of claim 2, wherein the pencil lead has a diameter in a rangeof 0.1-1.4 mm, and a length in a range of 20-70 mm.
 4. The modifiedgraphite electrode of claim 3, wherein the pencil lead is of a grade 6H,HB, B, or 4H.
 5. The modified graphite electrode of claim 1, wherein athickness of the coating is in a range of 0.001-5 μm.
 6. A method fordetecting uric acid, comprising: immersing a modified graphite electrodecomprising a graphite electrode with an outer surface and a coating onthe outer surface of the graphite electrode, wherein the coatingconsists of electrochemically reduced graphene oxide, and the modifiedgraphite electrode has an electroactive surface area in a range of0.03-0.6 cm², a reference electrode, and a counter electrode in anaqueous sample comprising 0.02-1,000 μM uric acid; and applying a squarewave potential to the aqueous sample.
 7. The method of claim 6, whereinthe reference electrode is Ag/AgCl, and the counter electrode comprisesplatinum.
 8. The method of claim 6, wherein the square wave potentialhas an amplitude in a range of 0.01-0.1 V, a frequency in a range of10-100 Hz, and an adsorption time in a range of 10-240 s.
 9. A devicefor sensing an analyte in a sample, comprising: a reference electrode; acounter electrode; and a modified graphite electrode comprising agraphite electrode with an outer surface and a coating on the outersurface of the graphite electrode, wherein the coating consists ofelectrochemically reduced graphene oxide, and the modified graphiteelectrode has an electroactive surface area in a range of 0.03-0.6 cm².10. The device of claim 9, wherein the reference electrode is Ag/AgCl.11. The device of claim 9, wherein the counter electrode comprisesplatinum.